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On Connections Between Dark Matter and the Baryon Asymmetry

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On Connections Between Dark Matter and the Baryon Asymmetry On Connections Between Dark Matter and the Baryon Asymmetry James Andrew Unwin Pembroke College, University of Oxford. Trinity Term 2013. A thesis submitted for the degree of Doctor of Philosophy. Abstract On Connections Between Dark Matter and the Baryon Asymmetry A thesis submitted for the degree of Doctor of Philosophy, Trinity Term 2013. James A. Unwin Pembroke College This thesis is dedicated to the study of a prominent class of dark matter (DM) models, in which the DM relic density is linked to the baryon asymmetry, often referred to as Asymmetric Dark Matter (ADM) theories. In ADM the relic density is set by a particle- antiparticle asymmetry, in direct analogue to the baryons. This is partly motivated by the observed proximity of the baryon and DM relic densities ΩDM ≈ 5ΩB, as this can be explained if the DM and baryon asymmetries are linked. A general requisite of models of ADM is that the vast majority of the symmetric component of the DM number density, the DM-antiDM pairs, must be removed for the asymmetry to set the DM relic density and thus to explain the coincidence of ΩDM and ΩB. However we shall argue that demanding the efficient annihilation of the symmetric component leads to a tension with experimental constraints in a large class of models. In order to satisfy the limits coming from direct detection and colliders searches, it is almost certainly required that the DM be part of a richer hidden sector of interacting states. Subsequently, examples of such extended hidden sectors are constructed and studied, in particular we highlight that the presence of light pseudoscalars can greatly aid in alleviating the experimental bounds and are well motivated from a theoretical stance. Finally, we highlight that self-conjugate DM can be generated from hidden sector particle asymmetries, which can lead to distinct phenomenology. Further, this variant on the ADM scenario can circumvent some of the leading constraints. Acknowledgements I wish to express my deepest thanks to John March-Russell for support, guidance, and inspiration. I am grateful to my collaborators Uli Haisch, Ed Hardy, Arthur Hebecker, Felix Kahlhoefer, and, especially, Stephen West for an enjoyable and pro- ductive time during my DPhil. I am indebted to Alan Barr, Joe Conlon, Rhys Davies, Savas Dimopoulos, Pavel Fileviez P´erez, Mads Frandsen, Peter Graham, Lawrence Hall, Joachim Kopp, Chris McCabe, Matthew McCullough, Apostolos Pilaftsis, Gra- ham Ross, Surjeet Rajendran, Francesco Riva, Raoul Rontsch, Joao Rosa, Subir Sarkar, Fidel Schaposnik, Laura Schaposnik, Kai Schmidt-Hoberg, David Shih, and Timo Weigand for insightful interactions. I would also like to thank Philip Candelas for acting as co-supervisor in the Mathematical Institute and my examiners Joe Conlon and C´elineBoehm for their constructive comments. I am thankful for the hospitality of the CERN theory group, the Institute for Theoretical Physics at Heidelberg University, the Max Planck Institute for Kernphysik (MPIK), Heidelberg, and the Institute for Theoretical Physics at Stanford Univer- sity during different periods, whilst undertaking work towards this thesis. I gratefully acknowledge that my DPhil research [1{8] has been funded by an EPSRC doctoral scholarship, a Charterhouse European Bursary, a Senior Scholarship at Pembroke Col- lege, awards from the Vice-Chancellors Fund and the Esson Bequest, and also through Stipendiary Lectureships at St. John's College and New College. Finally, I am grateful to my parents and grandparents for their unerring support. Statement of Originality This thesis is based on research undertaken between October 2010 and June 2013 and contains no material that has already been accepted, or is concurrently being submit- ted, for any degree or diploma or certificate or other qualification in this university or elsewhere. To the best of my knowledge and belief this thesis contains no material previously published or written by another person, except where due reference is made in the text. James A. Unwin June 2013 Contents 1 Introduction and Background1 1.1 The case for dark matter2 1.2 Dark matter production6 1.3 WIMP dark matter 11 2 Asymmetric Dark Matter 15 2.1 General features 15 2.2 Sharing and cogenesis of particle asymmetries 19 2.3 Aspects of ADM model building 21 3 The ADM Symmetric Component Problem 25 3.1 Relic abundance with an asymmetry 27 3.2 Effective operator connecting dark matter to quarks 29 3.3 Direct search constraints on contact operators 34 3.4 Discussion 44 4 Towards Successful Annihilation of the Symmetric Component: Light mediators 47 4.1 The light scalar-Higgs portal 48 4.2 The light pseudoscalar portal 52 4.3 Semi-analytic analysis of light mediators 54 4.4 Discussion 58 5 Towards Successful Annihilation of the Symmetric Component: Annihilation to hidden sector states 61 5.1 Efficient annihilation to hidden sector states 63 5.2 Model independent bounds on hidden sectors of ADM 65 5.2.1 Constrains on DM self-interactions via scalar mediators 67 5.2.2 Constrains on DM self-interactions via pseudoscalar mediators 69 5.3 Discussion 72 6 Evading the Symmetric Component Problem: The Exodus mechanism 75 6.1 Outline of the exodus mechanism 76 6.2 Constraints on the exodus mechanism 80 6.2.1 Lifetime constraints 80 6.2.2 Constraints on energy injection 84 6.3 Boltzmann equations for exodus 86 6.4 Exodus in the MSSM 90 6.5 Discussion 93 7 Summary and Closing Remarks 95 1 Introduction and Background This chapter roughly follows treatments in [9, 10]. The need for additional non-baryonic gravitating matter - dark matter - in or- der to explain astrophysical observations is generally accepted. Moreover, it is also widely held that the dark matter (DM) phenomenon is caused by the existence of new non-relativistic (meta) stable particle species with small or vanishing interactions with photons and hadrons. However, the precise nature of DM is one of the central questions in contemporary theoretical physics and (along with evidence for neutrino masses) is perhaps the most compelling indication of physics beyond the Standard Model (SM). This thesis is dedicated to the study of a prominent class of DM models, in which the DM relic density is linked to the baryon asymmetry, often referred to as Asym- metric Dark Matter (ADM). After outlining the main principles of both traditional DM and ADM in Chap.1&2, respectively, we proceed to carefully examine a crucial requirement on models of ADM in Chap.3. In Chap.4, we show that satisfying this the- oretical requirement leads to conflict with DM direct detection and collider constraints in a large class of models. We proceed to construct models which satisfy the constraints arising from direct detection and colliders in Chap.4&5, and we examine further ex- perimental limits which can constrain these models. In particular, we demonstrate that viable, motivated models of ADM can be constructed. In Chap.6 we outline a variant on the ADM scenario which circumvents many of the leading constraints on ADM and which can exhibit distinct phenomenology from traditional models. A summary of the thesis is presented in Chap.7 with some final remarks. 1 Introduction and Background 1.1 The case for dark matter The earliest indications of deviations in long range gravitational measurements were found by Fritz Zwicky in 1933 [11]. Zwicky estimated the total mass of the galaxy cluster Coma based on the motion of galaxies at the periphery of the cluster, and observed that his result deviated by two orders of magnitude from mass estimates based on the luminosity of the cluster and the number of constituent galaxies. Subsequently, he postulated that this large deviation between measurements was due to the presence of some non-luminous form of matter. Indeed, it was noted that the gravitational force due only to the visible matter would be insufficient for the galaxy cluster to be stable. Further, evidence for DM comes from observations of galactic rotation curves. If one examines bodies near the galactic edge, the trajectories deviate significantly from na¨ıve expectations of motion under the gravitational effects of just the luminous matter. Rather, the observed velocities of bodies in the periphery of the galaxy are consistent with motion under the gravitational potential generated by an approximately spherical halo of gravitating sources in addition to the observed disk. This lends significant weight to the idea of a DM halo which encompasses the visible galactic disk. Empirically, the gravitational potential can be inferred by measuring the velocities of bodies at different radial distances from the galactic centre. Take for instance galaxy NGC 6503, the galactic rotation curve for which is shown in Fig.1 (left) [12]. The plot shows rotational velocities as a function of radial distance for various bodies. Experimental data points are shown with 1σ error bars. The rotational velocity vc(r) can be calculated for a body at radial distance from the galactic centre r moving under a gravitational potential via Newtonian gravity r Z r GM(r) 0 02 0 vc(r) = ;M(r) = 4π ρ(r )r dr ; (1.1) r 0 - 2 - 1 Introduction and Background Figure 1. left. Galaxy NGC6503 rotation curve [12]. right. Bullet cluster [13]. Details in text. −2 19 where G = MPl is Newton's constant, with MPl = 1:2 × 10 GeV the Planck mass, and ρ is the mass density. The visible galactic disk, the DM halo, and the intergalactic gas present in the galaxy each contribute differently to the gravitational potential because of their dif- ferent mass distributions, as dictated by eq. (1.1). A three parameter fit of these components is used to match the data points and is shown in Fig.1 (left) as the solid line [12].
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